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Although the boiling-point method is able to dispose of a
greater number of convenient solvents than are suitable for
freezing-point determinations, it is never so accurate, mainly on
account of the difficulty of avoiding fluctuations in the boiling-
point, due to radiation, to the dripping of cold liquid from the
condenser, to impure solvent, and to barometric fluctuations.

Molecular Weight of Organic Acids                            '/{'
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Determination by means of the Silver Salt.—The               /j!|
basicity of an organic acid being known, the molecular weight                    (-ff
can be determined by estimating the amount of metal in one of                  is f
its normal salts.    The ratio of metal to salt will be that of the                   # *
atomic weight of the metal to the molecular weight of the salt.                   U\
The silver salts are usually selected for these determinations,                    iM]
since they are, as a rule, normal, i.e. neither acid nor basic ;                   ^ff
they are only slightly soluble in water, and are consequently                    ,/j^
readily obtained by precipitation, and finally they rarely contain                    |^|
water of  crystallisation.    On   the   other   hand  they are very                   1 j)
unstable, being quickly discoloured when exposed to light, and                   * >L
often decomposing with slight explosion when heated.    Silver
benzoate may be prepared by way of illustration.      Weigh out                       |f
roughly 2—3 grams of benzoic acid into a flask, and add about
20 c.c. of water and an excess of dilute ammonia.    Boil the
solution until the escaping steam has nearly lost the smell of
ammonia, and then test the liquid from time to time until it is
neutral to litmus. Cool the flask under the tap, and add an excess
of silver nitrate solution (3—4 grams AgNO3).     Filter with the
Filtration under Reduced Pressure.—A filter-pump is
an essential part of a laboratory fitting. It consists of a good
water-jet aspirator (see Fig. 35), which is fixed to the water-tap
by a stout piece of rubber tubing well wired at both ends. The
joint is wrapped round with cloth or leather wired on to the
rubber. The side tube of the aspirator is connected by pump
tubing to an empty filter flask or bottle by means of a glass tap.
A second glass tube or side piece is put in connection with the
filter flask by means of rubber tubing. The object of inserting
a vessel- between the pump and the filter flask is to prevent